Device and process for mode-locking a laser

ABSTRACT

The invention relates to a device for a laser blocked-mode, especially for a pulsed laser, comprising a cavity resonator ( 20 ) which is defined by a first mirror ( 1 ) and a second mirror ( 8 ), and fitted with an amplifying active laser medium ( 5 ) for the amplification of a beam of laser radiation of a fundamental frequency (ω1) and a solid, non-linear optic means ( 10 ) comprising at least said second mirror ( 8 ) for reversible conversion of the radiation of the fundamental frequency (ω1) into radiation of a harmonic frequency (ω2) whereby said non-linear optic means ( 10 ) has a reflection factor which increases with the intensity of the radiation of a fundamental frequency. The invention is characterized in that said device also comprises a solid intensity limiter ( 4 ) in the cavity resonator ( 20 ), whereby the transmission factor of the laser radiation decreases with the intensity of said radiation. The invention also relates to a method for a laser blocked-mode, especially for a pulsed laser, using said device.

CROSS-REFERENCE TO RELATED APPLICATIONS

Belgium 9900314 filed May 3, 1999.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMACT DISC

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device and to a process formode-locking a laser, and in particular a laser functioning in pulsedmode.

2. Description of Related Art

A laser cavity consists of an optical gain medium placed inside aresonator delimited by two mirrors oriented in auto-collimation, that isto say face to face. When the gain medium is activated, an opticaloscillation is maintained in the cavity, such that the device can emit alight beam characterized by a very high spatial and spectral brightness.

The mode-locking of a laser cavity consists in forcing short lightpulses to circulate in said resonator, so as to generate pulses of highpeak intensity and with a pulse length typically of less than 100picoseconds, which may be upto a few femtoseconds depending on the gainmedium used.

Lasers which may be distinguished include lasers of the continuous typein which the gain medium is permanently activated, that is to say overtime scales of from several seconds to several hours. A continuousmode-locked laser may thus generate short pulses at a repeat rate of theorder of a few tens to a few hundreds of megahertz, corresponding to thecirculation (to and fro) time of the pulses in the resonator.

This high repeat rate implies that a laser of this type will emitlow-energy light pulses. Nevertheless, this type of laser is adequatefor many applications which require a high mean optical power but whichcan make do with low pulse energy, such as the LIDAR technology, or“linear” absorption spectroscopy, photoionization spectroscopy,fluorescence spectroscopy, etc.

Moreover, lasers of pulsed type exist, which are characterized by a verylow working cycle of the gain medium (of less than 1/50). This gainmedium is activated for a short period, typically of less than onemillisecond at a low repeat rate typically of a few tens of hertz. Inpulsed mode, the gain medium may be temporarily very highly activated,corresponding to a large storage of optical energy in the gain medium,such that a mode-locked pulsed laser will be able to generate pulses ofmarkedly greater energy than those generated by mode-locked lasers ofcontinuous type. However, the fact, firstly, that the amplificationfactor of the gain medium is not constant during the transientactivation period, and, secondly, that the stabilization of the opticaloscillation in the laser cavity is a dynamic process which requires acertain amount of time and may thus be incomplete during the activationtime of the gain medium, limits the efficiency of the mode-locking andconsequently the brevity and energetic stability of said optical pulsesgenerated.

Pulsed lasers are used in manufacturing processes which requirehigh-energy optical pulses, such as for the ablation of materials, lasercutting and surface treatment, and also for “non-linear” opticalspectroscopies such as multi-photon resonant ionization or frequency-sumgeneration spectroscopy, and also any technique requiring a low repeatrate of the laser (time-resolved measurements).

One way for mode-locking lasers of pulsed type is to insert a cellcontaining a dye (liquid solvent), optionally combined with an intensitylimiter, into the laser cavity. This device has several drawbacks, inparticular:

-   -   the mobility and inhomogeneity of the solvent circulating in the        cell are factors causing energy instability of the emitted        pulses;    -   the chemical or photochemical degradation of said dye makes it        necessary for technicians to intervene regularly in order to        optimize the mode-locking process.

Document U.S. Pat. No. 4,914,658 describes a solid-state laser such as aNeodymium-doped Yttrium Aluminium Garnet (Nd:YAG) which is combined witha non-linear crystal and a dichroic mirror in order to create anon-linear optical means for mode-locking the laser. In the simplestembodiment of the device, the non-linear crystal makes it possible togenerate a beam at the second harmonic from the fundamental beamamplified by the gain medium. The oscillation in the resonant cavity ofthe portion of the fundamental beam not converted by the non-linearcrystal is negatively discriminated by means of a dichroic mirror whichmust have a reflection coefficient at the second harmonic frequencywhich is greater than that at the fundamental frequency.

Adjusting the optical distance between the non-linear crystal and thedichroic mirror makes it possible to obtain a suitable phase shiftbetween the fundamental beam and the beam at the second harmonic, so asto obtain an efficient reconversion of the beam at the second harmonicinto a fundamental beam in the non-linear crystal. This phase shift canalso be obtained by inserting a transparent plate between the non-linearcrystal and the dichroic mirror.

The non-linear optical means serves to increase the quality factor ofthe laser cavity, that is to say to reduce the energy losses of thelaser beam by reflection against the dichroic mirror, when theinstantaneous power of the beam at the fundamental frequency generatedby the gain medium increases. In other words, the non-linear opticalmeans induces a positive feedback on the quality factor of the lasercavity as a function of the instantaneous power of the beam at thefundamental frequency.

The non-linear optical mode-locking device is also characterized in thatthe ratio of the beam power at the second harmonic relative to the beampower at the fundamental frequency increases as the power of the beam atthe fundamental frequency increases.

Document EP-A-0 951 111 proposes a device and a method for mode-lockinga laser, preferably also working in continuous mode, which are based onthe principle described in document U.S. Pat. No. 4,914,658. In thiscase, it is proposed to convert part of the laser beam at thefundamental frequency into a beam at the second harmonic by using anon-linear crystal. The oscillation in the resonant cavity of the partof the fundamental beam not converted in the non-linear crystal isnegatively discriminated by means of the combination of a retardationplate and a polarizer. In said document, the gain medium is Nd:vanadate,the non-linear crystal is lithium triborate and the retardation platehas a retardation of λ/4=1064 nm and λ/2=532 nm. The retardation plateis placed between the non-linear crystal and the dichroic mirror, whilethe polarizer is placed between the gain medium and the non-linearcrystal.

It is pointed out in said document that the dichroic mirror, placedbehind the non-linear crystal, has a reflection coefficient at thesecond harmonic frequency which is not greater than the reflectioncoefficient at the fundamental frequency.

The non-linear optical means described in said document serves toincrease the quality factor of the laser cavity, that is to say, toreduce the energy losses of the laser beam by reflection against thepolarizer, when the instantaneous power of the beam at the fundamentalfrequency generated by the gain medium increases. In other words, thenon-linear optical means induces a positive feedback on the qualityfactor of the laser cavity as a function of the instantaneous power ofthe beam at the fundamental frequency.

The devices described in the two above-mentioned documents U.S. Pat. No.4,914,658 and EP-A 0 951 111 allow the efficient mode-locking ofcontinuous lasers. For example, pulses as short as ˜10 picoseconds FWHM(full width at half maximum) can be generated when an Nd:YAG gain mediumis used. However, these devices do not work properly in the case ofpulsed lasers. The shortest pulse widths ever obtained by means of thedevice as described in document U.S. Pat. No. 4,914,658 are 35picoseconds FWHM (full width at half maximum) in the case of a pulsedNd:YAG laser. These poor performance levels result from the fact thatthe gain factor of the active medium, the energy of the optical pulsesand thus the conversion yield for the non-linear crystal used in thenon-linear device vary greatly in the course of the activation period ofthe gain medium, which prevents any stabilization of the opticaloscillation in the resonant cavity. Moreover, the small number ofto-and-fro cycles within the cavity, and thus of interaction with thenon-linear device, produced by the optical pulses during the activationtime of the gain medium also limits the efficiency of the mode-locking.

BRIEF SUMMARY OF THE INVENTION

The present invention aims to provide a device and a process formode-locking a laser, making it possible to obtain particularly shortpulses and having great energy stability, even in the case of a pulsedlaser.

In particular, the present invention aims to overcome the drawbacks ofthe devices and processes of the prior art.

In particular, the present invention aims to propose a device consistingonly of solid components, and thus being simple to maintain whencompared with devices using dyes (liquid solvents), and which is robustand inexpensive. In addition, the various constituents used will show alow level of degradation over time.

The present invention relates to a device for mode-locking a laser, inparticular a laser of pulsed type, comprising a laser cavity delimitedby a first mirror and a second mirror, provided with an active mediumfor amplifying the laser beam at the fundamental frequency, and a solidnon-linear optical means which comprises at least said second mirror andwhich has a reflection coefficient which increases as the beam intensityincreases, characterized in that said device further comprises in thelaser cavity a solid intensity limiter whose transmission coefficient ofthe fundamental beam decreases as the intensity of said laser beamincreases.

More specifically, whereas the devices as described in document U.S.Pat. No. 4,914,658 and EP-A-0 951 111 display, by using a non-linearoptical means, only a positive feedback on the quality factor of thelaser resonant cavity as a function of the power of the fundamentalbeam, the device according to the present invention displays, by thecombined use of the non-linear optical means and the intensity limiter,both a positive feedback and a negative feedback on this quality factor.This is due to the fact that the non-linear optical means has areflection coefficient which increases as the intensity of thefundamental beam increases, whereas the intensity limiter has atransmission coefficient at the fundamental frequency of the laser whichdecreases as the intensity of the fundamental beam increases.

The combined use of the non-linear optical means and the intensitylimiter implies that the power ratio of the beam at the second harmonicrelative to the fundamental beam no longer increases as the intensity ofthe fundamental beam increases when said intensity exceeds the operatingthreshold of the intensity limiter.

Advantageously, the non-linear optical means comprises said secondmirror which corresponds to a dichroic mirror and a non-linear crystalable to convert the laser frequency.

The non-linear optical means may also comprise only said second mirror,which then corresponds to a Fabry-Perot anti-resonant saturable absorberconstructed from a superposition of dielectric or metallic semiconductorfilms.

The non-linear optical means may also comprise said second mirror whichcorresponds to a dichroic mirror, a frequency-converting non-linearcrystal and at least one polarizer.

Advantageously, the intensity limiter consists of a plate made of asemiconductor material such as GaAs, CdSe or InP.

Alternatively, the intensity limiter consists of a non-linear crystalwhich converts the fundamental beam into a beam at a harmonic frequency.

Alternatively, the intensity limiter consists of an active device, thatis to say an electronically controlled device which induces increasingenergy losses in the cavity when the intensity of the fundamental beamincreases, such as Pockels cell or an acousto-optical modulator.

Advantageously, the intensity limiter is arranged between the gainmedium and the non-linear optical means.

In a particularly advantageous manner, the intensity limiter and thenon-linear optical means are arranged on either side of the gain medium.

The present invention also relates to a process for mode-locking alaser, in particular a laser of pulsed type, which comprises:

-   -   emitting a laser radiation beam at the fundamental frequency by        stimulating an active laser medium,    -   converting the beam at the fundamental frequency into a beam at        a harmonic frequency,    -   returning the beam at the harmonic frequency to the resonant        cavity,    -   reconverting the beam at the harmonic frequency into a beam at        the fundamental frequency,        intensity limitation of the beam at the fundamental frequency        inside the resonant cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 describes one particular embodiment of the device for obtainingan Nd:YAG oscillator according to principle of the present invention.

FIG. 2 represents the pulse train envelope obtained for the Nd:YAGoscillator as described in FIG. 1.

FIG. 3 represents the measurement of the pulse width which is carriedout by a standard noise-free second-order auto-correlation.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 describes, by way of example, one embodiment of the deviceaccording to the invention. In a conventional manner, firstly, aresonant cavity 20 delimited by a first mirror 1 and a second mirror 8and, secondly, a non-linear optical means 10 comprising said secondmirror 8 are produced. The first mirror 1 is of high, preferably total,reflection, and the second mirror 8 is a dichroic mirror. Inside theresonant cavity 20 is arranged an active medium 5 which may, in aconventional manner, be an Nd:YAG (Neodymium-doped Yttrium AluminiumGarnet), Yb:YAG, Cr:YAG, Nd:YLF, Nd:glass, Ti:sapphire, Cr:forsterite orYb:glass medium. The medium is adapted to emit, under stimulation, laserradiation at a fundamental frequency ω1. The choice of such a medium isdictated by the desired wavelength of the laser and the desired spectralwidth of the gain.

According to one embodiment of the invention, the gain medium is a barof Nd:YAG crystal 5 with dimensions of 115×7 mm which is pumped by twoflash lamps for stimulating a laser beam at a fundamental frequencyω1=1064 nm.

The energy of the electric pump is −17 J, whereas the repeat frequencyis 20 Hz.

Two lenses 61 and 62 provided with an anti-reflection coating andcharacterized, respectively, by focal distances of 100 and −40 mm formthe telescope 6. A 0.8 mm diaphragm 3 limits the working of the laser toonly one transverse mode. The intensity limiter intended for the activemode-locking according to this embodiment comprises an AOML(acousto-optical mode-locker) component 2 located close to the mirror ofhigh reflection 1 and a GaAs plate 4. The total length of the cavity isapproximately 1.5 m and is adapted to the 100 MHz modulation frequencyof the AOML.

The non-linear optical means 10 comprises, in addition to the dichroicmirror 8, according to the embodiment represented in FIG. 1, anon-linear crystal 7 of BBO type with a length of 3 mm, for generating abeam at the second harmonic (ω2=532 nm) by a type I interaction. Thenon-linear optical means 10 has a reflection coefficient of greater than99% at 532 nm and equal to 25% at 1064 nm. Other non-linear crystals maybe used, such as LBO (lithium triborate), KDP (potassium dihydrogenphosphate), KTP (potassium titanyl phosphate), BBO (beta-barium borate),PPLN (periodically poled lithium niobate) or KNbO₃ (potassium niobate).The GaAs plate is aligned at the Brewster incident angle. Adjustment ofthe distance separating the non-linear crystal 7 of the dichroic mirror8 allows the phase shift between the fundamental beam and the beam atthe second harmonic to be controlled during the reconversion process.

When this distance is correctly adjusted, an appreciable increase in theintensity of the beam generated by the cavity is observed, which revealsthe efficient passive mode-locking of the YAG oscillator.

The mean output power of the laser cavity is −30 mW (pulse trainenergy=1.5 mJ) for an electric pump energy of −17 J when the AOMLcomponent 2 is used in the cavity.

FIG. 2 represents the 2 μs long pulse train envelope as measured by ap-i-n photodiode with an oscilloscope bandwidth of 60 MHz. The firstpart of the envelope (0–500 ns) is characterized by a rapid variation inthe pulse energy and is followed by a plateau from 600 to 1800 nscharacterized by a virtually constant pulse energy, estimated to be 10μJ/pulse.

Although previous studies revealed that this device could work withoutactive mode-locking, much more stable working of the YAG oscillator hasbeen observed when the AOML component is used in the cavity.

FIG. 3 shows the measurement of the pulse width at the centre of theplateau of the train envelope. This measurement is performed bynoise-free standard second-order auto-correlation by synchronizing the50 ns window of the auto-correlation signal integrator with the centreof the stable plateau of the train envelope.

Assuming a gaussian distribution, a pulse width of 12 ps FWMH is deducedfor the fundamental pulse. The peak intensity inside the cavity reachesa value of the order of 55 MW/cm², which is in accordance with the startof the two-photon absorption in a GaAs semiconductor.

In conclusion, it is possible to obtain a pulse width reduced to 12 psor even less using a pulsed Nd:YAG laser pumped with a flash lamp, bycombining a passive negative feedback component constituting theintensity limiter, which is a GaAs plate in the present case, with apositive feedback component, which is a non-linear optical means in thepresent case, consisting of a frequency-doubling non-linear crystal(BBO) coupled to a dichroic mirror.

The increase in the number of to-and-fro cycles performed by the opticalpulses and also their energy stabilization induced by the intensitylimiter are two key factors for obtaining short pulses. The width ofthese pulses is very close to the lower limit of ˜10 ps, set by theFourier transform of the gain spectrum of the Nd:YAG gain medium.

The pulsed laser equipped with the device described in the presentdocument has ideal characteristics for the synchronous pumping of anoptical parametric oscillator.

Moreover, the interposition, inside or outside the cavity, of passiveand active components for polarization selection and modification, suchas a Pockels cell, polarizers and retardation plates, will make itpossible to select energetic single pulses.

SEQUENCE LISTING

Not applicable

1. A device for a mode-locking laser, comprising a resonant cavity,delimited by a first mirror and a second mirror, provided with an activelaser gain medium arranged in the resonant cavity for amplifying a laserradiation beam at the fundamental frequency, and with a solid non-linearoptical means which comprises at least said second mirror, forreversible conversion of the radiation at the fundamental frequency intoradiation at a barmonic frequency, said non-linear optical means havinga reflection coefficient which increases as the intensity of theradiation at the fundamental frequency increases, said device furthercomprising a solid intensity limiter, arranged in the resonant cavity,whose transmission coefficient of the laser radiation passivelydecreases as the intensity of said radiation increases, wherein saidintensity limiter comprises a GaAs, CdSe or InP plate.
 2. The deviceaccording to claim 1, wherein the non-linear optical means correspondsto a dichroic mirror and a non-linear crystal that converts theradiation at the fundamental frequency into radiation at a harmonicfrequency.
 3. The device according to claim 1, wherein the non-linearoptical means comprises said second mirror which corresponds to adichroic mirror, a non-linear crystal that converts the radiation at thefundamental frequency into radiation at a harmonic frequency, and atlest one component for polarization selection and/or modification. 4.The device according to claim 2, wherein said non-linear crystal is aBBO crystal.
 5. The device according to one of claim 1, wherein thenon-linear optical means comprises only the second mirror, wherein saidsecond mirror corresponds to a Fabry-Perot anti-resonant saturableabsorber constructed from a superposition of dielectric or metallicsemiconductor films.
 6. The device according to claim 1, wherein theintensity limiter and the non-linear optical means are placed on eitherside of the active gain medium.
 7. The device according to claim 1,wherein the intensity limiter is placed between the nonlinear opticalmeans and the active gain medium.
 8. The device according to claim 1,wherein the active gain medium is an Nd:YAG crystal.
 9. The deviceaccording to claim 1, wherein the non-linear optical means has areflection coefficient of the radiation at the second harmonic which isgreater than the reflection coefficient of the radiation at thefundamental frequency.
 10. A device for a mode-locking a laser,comprising a resonant cavity, delimited by a first mirror and a secondmirror, provided with an active laser gain medium arranged in theresonant cavity for amplifying a laser radiation beam at the fundamentalfrequency, and a solid nonlinear optical means which comprises at leastsaid second mirror, for reversible conversion of the radiation at thefundamental frequency into radiation at a harmonic frequency, saidnon-linear optical means having a reflection coefficient which increasesas the intensity of the radiation at the fundamental frequencyincreases, wherein said device is provided with an intensity limitercomprising a GaAs, CdSe or InP plate with a transmission coefficientwhich passively decreases as the intensity of the radiation at thefundamental frequency increases, so as to ensure, in combination withsaid non-linear optical means, both a positive feedback and a negativefeedback on the quality factor of the resonant cavity.
 11. A process fora mode-locking a laser comprising: emitting a laser radiation beam atthe fundamental frequency by stimulating an active laser medium,converting the beam at the fundamental frequency into a beam at aharmonic frequency, returning the beam at the harmonic frequency to theresonant cavity, reconvening the beam at the harmonic frequency into abeam at the fundamental frequency, and passively limiting the intensityof the beam at the fundamental frequency inside the resonant cavity, bymeans of at least one GaAs, CdSe or InP plate.